1. Field of the Invention
The present invention relates to a loudspeaker for use in, for example, an audio system and, in particular, to a diaphragm of the loudspeaker.
2. Description of the Prior Art
As shown in FIG. 18, a loudspeaker includes a cone-type diaphragm 11. At a base end side (small-diameter side) of the diaphragm 11, a voice coil 12 is provided. By changing a magnetic field around the voice coil 12 depending on a sound signal, a magnetic force between a magnet (not shown) and the voice coil 12 is changed so as to vibrate the diaphragm 11 forward and backward, that is, along a center axis of the diaphragm 11. The diaphragm 11 is normally made of paper and formed into a conical shape. A large-diameter side opening edge (outer peripheral edge) 13 of the diaphragm 11 is held by an elastic ring (not shown) so that the diaphragm 11 can vibrate forward and backward. Since the vibration state of the diaphragm 11 controls a regenerative frequency characteristic, a high-frequency distortion frequency characteristic and the like, the performance of the loudspeaker is essentially determined by the diaphragm 11.
The ideal vibration is such that the diaphragm 11 makes forward and backward motions while maintaining its original conical shape. However, in practice, the diaphragm 11 presents behavior deviated from the ideal vibration. Recently, since the observation technique and the computer processing for the vibration state have been advanced, the actual vibration state has been largely elucidated. It has been known that, as shown in FIG. 19A, the diaphragm 11 may be twisted to cause corrugation of the large-diameter side opening edge 13, or, as shown in FIG. 19B, the circumference of the diaphragm 11 is corrugated to form nodes of the vibration while keeping axial symmetry. This behavior is called dividing vibration.
The reason for the behavior is considered as follows:
For example, immediately upon backward displacement of the diaphragm 11, since the large-diameter side opening edge 13 thereof tries to stay at the position due to inertia, it is resultantly contracted toward the center axis of the diaphragm 11. Thus, as shown in FIG. 20, compressive forces are generated at any circumferential portions of the diaphragm 11. In contrast with this, immediately upon forward displacement of the diaphragm 11, tensile forces are generated at any circumferential portions of the diaphragm 11.
Accordingly, as described above, the ideal behavior is not accomplished to thereby degrade the sound quality. The conventional loudspeaker has the basic problem as noted above, which thus causes the following problems:
For ensuring sufficient sound pressures at a low-frequency region, the diaphragm 11, which is thick and large, is required. Thus, the diaphragm 11 becomes heavy to increase its moment of inertia. The dividing vibration becomes greater as the moment of inertia becomes greater or the vibration frequency becomes greater (as the frequency of occurrences of crests and troughs on the circumference of the diaphragm 11 becomes greater). Eventually, the loudspeaker incorporating such a diaphragm 11 can be only used at the low-frequency region.
On the other hand, for using the diaphragm 11 at a high-frequency region, since an influence of the dividing vibration is large, the moment of inertia is required to be small. Thus, the diaphragm 11, which is small in thickness and large in strength, is required. Even if the diaphragm 11 is thin, the sufficient sound pressures can be ensured at the high-frequency region. However, in this case, the loudspeaker incorporating such a diaphragm 11 can not be used at the low-frequency region, and further, it is necessary to use titanium, beryllium or the like, which is expensive, as a material of the diaphragm 11.